Traditional laminated panels, intended to be used for, e.g. flooring or furniture components, are produced by the following steps as shown in FIGS. 1a-1d. A decorative paper 2b and a wear resistant transparent overlay paper 2a are impregnated with a thermosetting resin, such as melamine, and are applied on the upper part of a HDF core 3. A balancing paper 4 impregnated with a melamine resin, general called backing, is applied on the backside of a HDF core. The core 3 with the upper 2 and lower 4 layers is moved into a press 5 and pressed under heat and pressure such that the thermosetting resins are cured and the layers are attached to the core as shown in FIG. 1b. 
Typical press parameters are 40 bar pressure and a temperature of 160-200 C.° with a pressing time of 12-30 seconds.
This production method and product produced by such methods are generally referred to as the DPL process and DPL products (Direct Pressure Laminate) The upper and lower surface layers have generally at thickness of 0.1-0.2 mm
HDF (High density fibreboard) comprises wood fibre and a thermosetting resin that also is cured by heat and pressure to a board with a thickness of about 6-12 mm and a density of about 800 kg/m3.
The most common floor size is a rectangular panel of 1.3*0.2 m with a thickness of about 8 mm. The panels are packed and supplied in a packet that contains about 10 panels with a floor area of about 2 m2. The weight of each packet is about 16 kg.
Recently new floor panels with a wood powder based surface and backing have been developed. The paper is replaced with a powder backing 4 comprising wood fibres and melamine particles that is scattered on one side of a core 3 and a powder based surface layer 2 comprising wood fibres, thermosetting resins, preferably melamine particles, aluminium oxide particles and colour pigments, is scattered on the other side of the core that generally is a HDF board. The scattering is made by rollers and brushes and very accurate layers of about 100-800 gr/m2 may be scattered with high precision on the HDF core material that generally has a thickness of about 7-10 mm. The surface 2, the core 3 and the backing 4 are pressed under heat and pressure in a continuous or discontinuous press 5 to obtain a product with a paper free and solid surface layer and backing.
The pressed powder based layers may have a thickness of about 0.2-1.0 mm. Typical press parameters are similar to conventional laminate flooring and may be a pressure of 40-80 bar and a temperature of 160-200 C.° with a pressing time of 10-40 seconds.
Such wood fibre based floors, generally referred to as WFF floors, have considerably better properties than traditional laminate floors since a thicker and more impact and wear resistant surface may be produced in a cost efficient way.
These two production methods may be combined.
A laminate floor with a paper based surface layer may have a powder based sub layer under the decorative paper in order to provide better impact resistance and deeper embossing. The paper backing may be replaced with a powder backing. The sub layer may be used to impregnate the decorative paper during pressing when the resins from the sub layer penetrate into the decorative paper.
WFF floor may also have several different layers on the upper side for example a high quality top layer 2a and a more cost efficient sub layer 2b under the top layer. The sub layer may comprise lower resin content and no aluminium oxide particles are needed.
A common feature for the paper or powder based surface layers, the paper or powder based backing layers and the HDF core is that all these materials comprise wood fibres and thermosetting binders, preferably melamine or urea, and that they are cured by heat and pressure. The wood fibres may be of the same type.
The layers are exposed to a first shrinking when the thermosetting resin in the upper and lower layer cures during pressing. The HDF core is also heated and becomes soft and easy to bend. The backing layer balances the tension that is created by the surface layer and the panel is substantially flat with a small convex backward bending when it leaves the press. The second temperature shrinking, when the panels is cooled from about 160-200° C. to room temperature, is also balanced by the backing layer and the panel 1 is essentially flat. A small convex backward bending is preferred since this counteracts upward bending of the edges in dry conditions when the relative humidity may go down to 20% or lower during wintertime.
This essentially flat pressed board comprises tension forces caused by the shrinking of the surface and balancing layers.
The board is generally cut and formed into several floor panels with locking systems on long and short edges as shown in FIG. 1c. The locking system comprises generally a tongue 10 and a tongue groove 9 for vertical locking and a strip 6 with a locking element 8 that cooperates with a locking groove 14 for horizontal locking.
The surface layer 2 has about the same length and width as the backing layer 4 as shown in FIG. 1d. The locking system may be formed in one piece with the core. Alternatively separate materials may be used to form for example the tongue 10 and/or the strip 6.
The prices for wood fibres and thermosetting resins are increasing and major increases are expected in the future due to shortage and the possibility to use wood fibres for energy production.
Several methods have been used to save material and to reduce costs. Such methods are mainly aiming to make thinner products that comprise a minimum of resins. Further cost savings are limited by minimum quality requirements related to the floor panel and the geometry of locking system.
It would be a major advantage if it would be possible to reduce the weight and material content. The problem with the present laminate and WFF floors is that they must have a high density core such as HDF that is needed for the stability, impact resistance and the strength which is needed to resist the heat and pressure from the pressing operation. Another problem is that the panels must have a minimum thickness and a core with high shear strength in order to allow the forming of a locking system with sufficient strength and geometry that allows easy installation.
It is known that grooves may be formed on the rear side of solid wood floors mainly in order to increase the flexibility of the panel. Such panels are easier to glue down to the sub floor. Grooves at the rear side of the panels are not used in laminate and WFF floors, which are installed with mechanical locking systems. The main reason is that such groove will have a negative impact on the stability of the panel and on the locking system since material will be removed from the balancing layer and the lower parts of the locking system.